Despite almost 300,000 percutaneous transluminal coronary angioplasty procedures per year, restenosis complicates about one third of cases by 6 months. The mechanism(s) responsible for restenosis and appropriate therapeutic approach(es) to restenosis remain unknown. Restenosis is thought to result from a complex interaction of platelet-rich thrombus formation, release of vasoactive and mitogenic factors, and migration and proliferation of smooth muscle cells in the intimal layer of the dilated artery. Recent studies suggest the potential important role of thrombin in vessel healing following injury, and may contribute to restenosis. Consistent with this hypothesis we have recently demonstrated the effectiveness of recombinant hirudin (r-hirudin), a thrombin inhibitor, in limiting restenosis after balloon angioplasty in our rabbit model. The overall goal of this project is to ascertain the mechanism(s) by which thrombin may promote neointimal proliferation following balloon angioplasty and how r-hirudin, a potent and specific inhibitor of thrombin, results in a lower restenosis rate. Aim 1 will determine whether the observed reduction in neointimal proliferation after balloon angioplasty using r-hirudin correlates with its known potent inhibition of mural thrombosis in-vivo. Aim 2 will ascertain whether the r-hirudin- induced reduction in neointimal proliferation results from inhibition of smooth muscle cell migration and/or proliferation. Aim 3 will evaluate thrombin as a mitogen. Cultured endothelial and smooth muscle cells will be used to determine whether thrombin acts as a direct mitogen or potentiates the effect of other known mitogens. The mechanism(s) by which r-hirudin blocks these mitogenic effects will be determined. Aim 4 will use a short-term whole organ culture system to investigate the growth modulation of smooth muscle cells identified in Aim 3 under conditions that are much more representative of those in-vivo. Taken together, these studies will provide important novel insights into the role of thrombin in vessel healing following balloon angioplasty and should identify the mechanism(s) by which r-hirudin limits restenosis. This work may have considerable impact clinically with respect to enhancing the effectiveness of angioplasty, reducing morbidity and limiting overall cost.